A 500 MHz NMR spectrometer for use by the structural biology community at Cornell University is requested. The system includes four radio frequency channels, a 5.1 cm bore superconducting magnet, and associated electronics and software to allow implementation of modern triple resonance experiments that employ deuterium decoupling, pulsed field gradients (pfg), and selective pulses. One probe is requested: an inverse triple resonance 1H [13C, 15N, 31P] probe (5 mm, with Z-axis pfg). The spectrometer would become part of the Cornell Biomolecular NMR Center, which is dedicated to structural and dynamic studies of biological macromolecules. The Biomolecular NMR Center houses a single Varian Inova 600 spectrometer, which is the only high field spectrometer at Cornell specifically dedicated to structural biology. The success of this facility has generated unprecedented demands for spectrometer time. Hence, the requested 500 MHz spectrometer is required to keep pace with the needs of the users. It is also vital for studies of protein dynamics (providing an additional field strength necessary for characterizing for characterizing complex motions), and for implementation of the latest advancements in structure determination techniques that exploit the field dependence of residual dipolar couplings and chemical shift in partially aligned proteins in solution. The spectrometer will be used by at least ten laboratories at Cornell involved in projects designed to reveal fundamental principals that govern interactions between proteins and their biological targets. Several proteins under investigation are involved in signal transduction and/or intracellular trafficking, including the GTP-binding protein Cdc42Hs and its cellular targets (Oswald, Cerione), the regulatory apparatus of c-Src (Nicholson, Shalloway), and the cytoplasmic tail of the amyloid precursor protein (Nicholson). Other projects focus on mechanisms of enzyme catalysis and inhibition, including thiamine sarcoma virus protease mutant (Vogt, Nicholson). Protein folding is being investigated in studies of ribonuclease A (Scheraga). Finally, molecular recognition is being studied by characterizing interaction between thrombin and fragments of fibrinogen (Scheraga), and between natural products and DNA (Rodriguez). The proposed research projects will fully utilize both the existing 600 and the proposed 500 MHz spectrometers, and will exploit the advantages of access to two different field strengths.